Glutamate is the primary excitatory neurotransmitter in the mammalian central nervous system. Glutamate is released by presynaptic neurons and signals primarily through postsynaptic ionotropic and metabotropic receptors. Excitatory amino acid transporters (EAATs) play critical roles in neuronal physiology and pathophysiology. In order for high frequency signaling to occur, neurotransmitter must be removed from the synapse soon after release. Additionally, low levels of ambient glutamate are required to prevent chronic excitotoxic activity of high-affinity receptors. In pathological circumstances, changes in glutamate transporter function and/or surface density can occur. Disruption of alkali ion gradients caused by metabolic impairment after stroke or traumatic brain injury cause transporter slowing or reversal, contributing to glutamate elevation and excitotoxicity. Bidirectional changes in transporter surface expression occur in both normal and pathophysiological conditions, causing ambient glutamate levels to change. Development of selective new ligands for modulating and imaging glutamate transporter density changes is an important goal. We propose to determine the first high-resolution atomic structure of a human neuronal glutamate transporter. This will provide new understanding of transporter function and facilitate development of selective and high- affinity EAAT ligands for potential therapeutic and diagnostic uses.